1. Technical Field
This technology is related to thin-film lithium-based batteries and electrochromic devices and to methods and materials for fabricating same.
2. Prior Art
Thin-film lithium-based batteries usually have several electrochemical cells that are connected in series and/or parallel to provide a desired voltage and capacity. Each cell has a positive electrode (cathode) and a negative electrode (anode) separated by an electrolyte solution containing dissociated lithium salts, which enable lithium ion (Li+) transfer between the two electrodes. Both of the electrodes are sources of chemical reactions, and once the electrodes are connected externally, the chemical reactions proceed in tandem at both electrodes, which causes electrons to flow through the external connection from the negative electrode (anode) to the positive electrode (cathode). If an electrical load is placed in the external connection (circuit) between the anode and the cathode, the flow of electrons can produce useful work.
Myriad materials are being, or have been, used for the positive electrodes, negative electrodes, and electrolytes in lithium ion battery cells, as explained, for example, in the article J. M. Tarascon & M. Armand, “Issues and challenges facing rechargeable lithium batteries,” Nature, Vol. 415, pages 359-367, 15 Nov. 2001. Several of such materials used for positive electrodes include, for example, vanadium oxides, lithium-metal-oxides, such as LiNiO2, LiCoO2, LiNi1-xCOxO2, and various di-, tri-, or tetravalent substitutes for Ni or Co (e.g., Al, Ga, Mg, or Ti), LiMnO2, LiFeO2, and others. Several of the materials used for negative electrodes include lithium metal (Li), carbons (e.g., graphite), lithium transition-metal nitrides, and others. A number of aqueous and non-aqueous electrolytes, including gels and solids, have been used, some of the more common of which include perfluorosulphonimide Li+[CF3SO2NSO2CF3]− salt (LiTFSI) and glassy lithium phosphorus oxynitride (LiPON), such as Li3PO4-xNx, where x can be any number between 0 and 4 that makes a stoichiometric material.
Lithium ion cells can also be made with positive and/or negative electrodes that have electrochromic characteristics, i.e., change in transmissivity depending on applied electric field. Consequently, lithium ion cells have also been adapted to electrochromic devices, for example, windows and other glazing, lighting and displays, optical communications systems, architectural materials, analytical instruments, and consumer goods. Electrochromic lithium ion cells have similarities to lithium ion battery cells, including, for example, a lithium salt electrolyte between chemically reactive electrodes. However, battery cells do not have to be transparent, whereas electrochromic windows and other devices have to be at least somewhat transmissive to light at least in the bleached phase. State-of-the-art electrochromic devices utilize two electrochromic materials, one of which—the negative electrode (also called “cathode,” “working electrode,” or “active electrode”)—changes from a colorless (bleached) state to a colored (darkened) state upon reduction (gain of electrons) while the other of which—the positive electrode (also called the “anode” or “counter electrode”)—also changes from a colorless (bleached) state to a colored (darkened) state upon oxidation (loss of electrons). The reduction of the working electrochromic material at the negative electrode and the oxidation of the counter electrochromic material at the positive electrode are forced by an external power source, which operates as an electron pump to transfer electrons from one electrode to the other. Reversal of the voltage polarity reverses the coloration to re-bleach both the negative electrode material and the positive electrode material. The current is transferred internally by the lithium ions (Li+) in the electrolyte. Examples of electrochromic materials useful for the negative cathode include tungsten and niobium oxides, but for electrochromic windows, tungsten oxide (WO3) has become the material of choice because of its superior electrochromic properties and relative ease of deposition by sputtering and a variety of other methods. Examples of electrochromic materials suitable for the positive electrode include oxides of nickel, manganese, cobalt, molybdenum, iron, and vanadium as well as LiNiO2, LiMnO, and LiFeO2. LiPON is one of a number of lithium ion electrolytes that can be used in lithium ion electrochromic devices.
In spite of all of the developments and improvements in lithium ion batteries and electrochromic devices, improvements in cycle life, lifetime, cell potential, and capacity (charge density) for batteries and in response time, photopic transmittance ratio, and service lifetime for electrochromic devices are still desired.